This invention relates to a method for depositing a layer on the inside of cavities of a work piece and particularly, to the chemical deposition of such a layer from the gaseous phase.
In the construction of chemical apparatus, work pieces are often required which have a very high resistance to corrosive media such as acids. Although glass or plastic meet this criterion, if such work pieces are provided for use at high pressures, such materials can not be employed for such apparatus or for piping systems, and metallic materials such a Hostalloy C or tantalum must be used instead. However, these materials are very expensive and are difficult to work. To reduce the manufacturing costs, thought has been given to the possibility of making such liquid-carrying or gas-carrying piping systems of steel parts which are plated on the inside with a corrosion protection layer, for example, of tantalum. Plating such hollow parts, which often have different inside diameters and, for example, several separately disposed inside canals and openings, can usually be accomplished only by precipitation from the gaseous phase (CVD deposition), since materials such as tantalum, which are suitable for corrosion protection, cannot be electrodeposited. Such CVD deposition methods are known, for example, from U.S. Pat. No. 3,127,641, in which a method for the deposition of tungsten on the inside of a copper tube is described. According to another known method for the inside coating of work pieces, a work piece to be plated is brought into a suitable susceptor, and the deposition is accomplished by means of a CVD method using electric induction heating for heating the work piece. Further information on the chemical gaseous-phase deposition of different materials, such as Ta, W, Mn, Si, Ti, TiC, CrC is given in the journal "Thin Solid Films", 24 (1974), pages 157 to 164.
Inside coating of complicated and difficult-to-manufacture parts, such as, for example, multi-path valves and pump housings which have more than two openings and several internal canals, has not been possible to date with satisfactory results, since the deposited layers exhibited uneven thickness and, in part, also open places. For this reason such parts have been made of solid material, for example, solid tantalum. Such fabrication is very expensive, and, in addition, it must further be taken into consideration that certain work pieces can be made of steel but not, for example, of a material such as tantalum.